A number of aircraft accidents occur each year as a result of "wind shear". The nature of windshear has become better known in the last year or two, and a particularly dangerous form of wind shear involves a meterological phenomenon known as a "downburst" or a "microburst". These are particularly insidious and dangerous forms of wind shear, as they are highly localized, often 2 to 3 kilometers in diameter; they are short-lived, in the order of two to six minutes in duration; and often occur in what has appeared visually to be benign meterological conditions.
A "downburst" or "microburst" is a localized, intense downdraft with vertical currents exceeding a downward speed of 12 feet per second (720 feet per minute) at about 300 feet above the surface of the ground. As the "downburst" or "microburst" strikes the ground, a gust front is created by the horizontal outflow, and this can be visualized by analogy to pouring a thick liquid onto a table top. The outward flow pattern can have velocities in the order of 40 to 70 knots (in the order of 46 to 80.5 miles per hour). Also, in traversing the microburst near the ground, the velocity shifts from plus 40 to 70 knots (a headwind) to minus 40 to 70 knots, (a tailwind) with a resultant shift in wind velocity relative to the ground of 80 to 140 knots.
Incidentally, the precise meteorological factors going into the formation and development of microbursts or downbursts are not known with assurance; however, it appears possible that it may involve special preconditions where warm moisture in the air turns into water vapor and expands and rises rapidly in a cumulative build-up of upsurging air; and the "downbursts" or "microbursts" may be an aftermath or counter-flow of down surging air. However, for the purposes of the present invention the exact nature of the origins of the phenomenon are not really important. Additional background information on "microbursts" or "downbursts" is set forth in an article entitled "What You Don't Know About Wind Shear Can Kill You", by William Melvin, which appeared in the August 1982 Delta Air Lines Publication, "The Roar" published by Delta Air Lines Local Council, No. 44 Atlanta, Ga., page 2 et seq.
One particularly dangerous form of encounter with wind shear is shown in FIG. 1 of the drawings to be described in greater detail below. The aircraft has just taken off and is starting to climb, as it enters the "microburst" or "downburst". At this point the aircraft is flying in a strong head wind, perhaps 40 to 70 knots. Then it passes through the center of the microburst and suddenly is in a strong tail wind. Since the inertia of the aircraft maintains its speed constant with respect to the earth, the sudden change in wind direction results in a loss of airspeed possibly below stall conditions, with possible serious consequences.
More specifically, this type of wind shear characteristically occurs at low altitudes; and when an aircraft encounters these "microbursts" or "downbursts" it is normally either taking off and climbing or is in an approach for landing. For a Boeing 727 aircraft in these situations, for example, the air speed is approximately 140 knots and the stall speed is about 105 to 110 knots. Thus, a 40 knot change in air speed due to shear will put the airplane into a stall. When this occurs unexpectedly, there may be insufficient altitude to recover.
Ground based equipment to measure and warn of wind shear in the vicinity of airports have been tried with varying degrees of success (see reference 2). These devices range from mechanical anemometers placed at various locations on the airport, to sophisticated doppler weather radars (see reference 4) and more recently laser radars (LIDAR) (see reference 5). The simpler systems are inadequate because they can miss the presence of wind shear completely if it occurs outside of the immediate vicinity of the airport. The doppler radars and LIDARS can cover a reasonable area surrounding the airport; however, these systems have the disadvantage of being expensive, and destined for installation in a relatively few major airports. Hence, a large number of airports served by the airlines may never have a wind shear detection system. Secondly, the delay time in getting the information from the controller to the pilot may be too long for corrective action to be taken. Finally, a given wind shear field affects different types of aircraft differently. Therefore, the pilots' corrective actions depend on the character of the shear and type of aircraft. This latter type of information would be difficult to pass from controller to pilot.
Various airborne systems have been installed and tested. Examples of such systems are instruments that compare, ground speed to airspeed, devices that give warning based on comparison between angle of attack and pitch attitude, and laser devices. These schemes, as applied, do give warning of wind shear. However, because of the principles upon which they are based or because of implementation, the warning time is too short (0-3 seconds) and the pilot recognizes he's in wind shear at about the same time he gets a warning.
Accordingly, a principal object of the present invention is to provide a wind shear detection and prediction system which will allow adequate time for appropriate action to be taken to avoid the types of aircraft disasters which have all too often occurred when aircraft encounter wind shear phenomena, particularly of the microburst or downburst type.